Parsec (pc) - Unit Information & Conversion

Quick Answer

What is a Parsec? A parsec (pc) is the astronomical distance at which Earth's orbit (1 AU) appears to span one arcsecond (1/3600°). It equals approximately 3.26 light-years, 31 trillion kilometers, or 19 trillion miles. Parsecs measure distances to stars and galaxies—the nearest star (Proxima Centauri) is 1.3 parsecs away, the Milky Way's center is 8,000 parsecs (8 kiloparsecs), and nearby galaxies are millions of parsecs (megaparsecs) distant. Use our length converter for parsec conversions.

Quick Comparison Table

Unit	Distance in Meters	Relative to 1 pc
1 parsec (pc)	3.0857 × 10¹⁶ m	1× (baseline)
1 light-year (ly)	9.4607 × 10¹⁵ m	0.3066×
1 astronomical unit (AU)	1.496 × 10¹¹ m	4.848 × 10⁻⁶×
1 kiloparsec (kpc)	3.0857 × 10¹⁹ m	1,000×
1 megaparsec (Mpc)	3.0857 × 10²² m	1,000,000×
1 gigaparsec (Gpc)	3.0857 × 10²⁵ m	1,000,000,000×
1 kilometer (km)	1,000 m	3.241 × 10⁻¹⁴×
1 light-second	2.998 × 10⁸ m	9.716 × 10⁻⁹×

Quick context: Proxima Centauri, the nearest star, is 1.3 parsecs away. A laser beam traveling at light speed takes 4.2 years to cross that distance. The Andromeda Galaxy is 770,000 parsecs (0.77 megaparsecs) away.

Definition and Standards

Geometric Definition

The parsec is defined through trigonometric parallax:

1 parsec = the distance at which 1 astronomical unit (AU) subtends an angle of 1 arcsecond (1″)

Mathematically:

• 1 parsec = 1 AU / tan(1″)
• Since 1″ = 1/3600 degree = π/648,000 radians ≈ 4.8481 × 10⁻⁶ radians
• For small angles: tan(θ) ≈ θ (in radians)
• 1 parsec ≈ 1 AU / 4.8481 × 10⁻⁶ ≈ 206,265 AU

Exact IAU Value

The International Astronomical Union (IAU) defines the parsec exactly as:

1 parsec = 648,000/π AU ≈ 206,264.806247 AU

Using the IAU-defined astronomical unit (1 AU = 149,597,870,700 meters exactly as of 2012):

1 parsec = 30,856,775,814,913,673 meters (exactly)

Or approximately:

• 3.0857 × 10¹⁶ meters
• 30.857 trillion kilometers
• 19.174 trillion miles

Relationship to Light-Year

The light-year (distance light travels in one Julian year) relates to the parsec:

1 parsec ≈ 3.26156 light-years

More precisely: 1 pc = 3.261563777 ly (using Julian year of 365.25 days)

Standard Multiples

Kiloparsec (kpc): 1 kpc = 1,000 pc ≈ 3,262 ly

• Used for distances within galaxies
• Milky Way diameter: ~30 kpc

Megaparsec (Mpc): 1 Mpc = 1,000,000 pc ≈ 3.26 million ly

• Used for intergalactic distances
• Andromeda Galaxy: ~0.77 Mpc

Gigaparsec (Gpc): 1 Gpc = 1,000,000,000 pc ≈ 3.26 billion ly

• Used for cosmological distances
• Observable universe radius: ~14 Gpc

History and Evolution

Pre-Parsec Era: The Parallax Quest (1600s-1830s)

The concept of stellar parallax dates to ancient Greek astronomy, but detecting it required centuries of technological advancement.

Galileo Galilei (1610) suggested that if Earth orbits the Sun, nearby stars should show annual parallax shifts against distant background stars. No parallax was detected, leading geocentrists to argue Earth must be stationary.

James Bradley (1728) discovered stellar aberration (apparent star position shifts due to Earth's orbital motion combined with finite light speed), confirming Earth's motion but still failing to detect parallax—stars were simply too distant.

Friedrich Wilhelm Bessel achieved the first successful parallax measurement in 1838 for 61 Cygni, determining a distance of about 10.3 light-years (3.16 parsecs, though the term didn't exist yet). This triumph came using a heliometer—a split-lens telescope enabling precise angular measurements.

Thomas Henderson measured Alpha Centauri's parallax (1832-1833, published 1839), and Friedrich Struve measured Vega's (1837), establishing parallax as the fundamental distance measurement method.

Coining the Term (1913)

Herbert Hall Turner (1861-1930), British astronomer and director of Oxford University Observatory, coined "parsec" in 1913. Before this, astronomers expressed stellar distances awkwardly:

• In astronomical units (requiring numbers in the hundreds of thousands)
• In light-years (popular but not directly tied to measurement method)
• In "parallax seconds" (inverse of parallax angle, but confusing terminology)

Turner recognized that astronomers naturally thought in terms of parallax angles. For a star with parallax angle p (in arcseconds), the distance d is simply:

d (in parsecs) = 1 / p (in arcseconds)

This elegant relationship made the parsec immediately practical. A star with 0.5″ parallax is 2 parsecs away; 0.1″ parallax means 10 parsecs; 0.01″ parallax means 100 parsecs.

IAU Adoption (1922-1938)

The 1922 IAU General Assembly in Rome endorsed the parsec as the standard unit for stellar distances, though adoption wasn't immediate or universal.

The 1938 IAU General Assembly in Stockholm formally standardized the parsec definition based on the astronomical unit and arcsecond, solidifying its status.

By the 1950s, the parsec dominated professional astronomy literature, though popular science continued preferring light-years for general audiences.

Space Age Precision (1960s-Present)

Hipparcos satellite (1989-1993): European Space Agency mission measured parallaxes for 118,000 stars with milliarcsecond precision, extending reliable parsec-based distances to hundreds of parsecs.

Gaia mission (2013-present): ESA's Gaia spacecraft has revolutionized astrometry, measuring parallaxes for 1.8 billion stars with microarcsecond precision. This extends direct parsec measurements to 10,000+ parsecs (10+ kiloparsecs), mapping our galaxy's structure in unprecedented detail.

2012 IAU redefinition: The IAU redefined the astronomical unit as exactly 149,597,870,700 meters (no longer based on Earth's actual orbit, which varies slightly). This made the parsec exactly 648,000/π AU, providing a stable definition independent of Earth's orbital variations.

Real-World Examples

Nearby Stars (1-10 parsecs)

• Proxima Centauri: 1.301 pc (4.24 ly) - nearest star to Sun
• Alpha Centauri A/B: 1.339 pc (4.37 ly) - nearest Sun-like stars
• Barnard's Star: 1.828 pc (5.96 ly) - second-nearest star system
• Sirius: 2.64 pc (8.60 ly) - brightest star in night sky
• Epsilon Eridani: 3.21 pc (10.5 ly) - nearest Sun-like single star
• Tau Ceti: 3.65 pc (11.9 ly) - similar to our Sun
• Procyon: 3.51 pc (11.46 ly) - eighth brightest star
• Altair: 5.13 pc (16.73 ly) - one of closest naked-eye stars

Stellar Neighborhoods (10-1,000 parsecs)

• Vega: 7.68 pc (25.04 ly) - bright summer star
• Betelgeuse: ~168 pc (550 ly) - red supergiant in Orion
• Rigel: ~264 pc (860 ly) - blue supergiant, Orion's "foot"
• Antares: ~170 pc (550 ly) - red supergiant in Scorpius
• Deneb: ~800 pc (2,600 ly) - most distant first-magnitude star
• Pleiades cluster: 136 pc (444 ly) - famous open cluster
• Hyades cluster: 47 pc (153 ly) - nearest open cluster

Galactic Scale (kiloparsecs, kpc)

• Sun to galactic center: ~8.2 kpc (~27,000 ly)
• Milky Way disk diameter: ~30 kpc (~100,000 ly)
• Milky Way disk thickness: ~0.3 kpc (~1,000 ly)
• Magellanic Clouds: 50-63 kpc (163,000-206,000 ly) - Milky Way satellite galaxies
• Galactic halo extent: ~60 kpc (200,000 ly)
• Sagittarius Dwarf Galaxy: ~24 kpc (~78,000 ly) - colliding with Milky Way

Extragalactic Distances (megaparsecs, Mpc)

• Andromeda Galaxy (M31): 0.77 Mpc (2.5 million ly) - nearest large galaxy
• Triangulum Galaxy (M33): 0.84 Mpc (2.73 million ly) - third-largest Local Group member
• Local Group diameter: ~3 Mpc (10 million ly)
• Virgo Cluster center: ~16.5 Mpc (54 million ly) - nearest galaxy cluster
• Coma Cluster: ~100 Mpc (330 million ly)
• Most distant galaxy observed: ~4,200 Mpc (13.7 billion ly) - near edge of observable universe

Cosmological Scales (gigaparsecs, Gpc)

• Hubble length (c/H₀): ~4.4 Gpc (14.4 billion ly) - characteristic cosmological distance
• Particle horizon (observable universe radius): ~14.3 Gpc (46.5 billion ly)
• Comoving distance to CMB: ~14 Gpc (46 billion ly)

Common Uses

Stellar Astronomy and Parallax Measurements

The parsec's primary use is measuring stellar distances via trigonometric parallax:

Parallax formula: d (parsecs) = 1 / p (arcseconds)

Ground-based observatories: Measure parallaxes to ~0.01″ accuracy, reliable to ~100 pc

Hipparcos satellite: Measured parallaxes to ~0.001″ (1 milliarcsecond), reliable to ~1,000 pc (1 kpc)

Gaia spacecraft: Measures parallaxes to ~0.00001″ (10 microarcseconds) for bright stars, reliable to ~10 kpc for many stars

Applications:

• Calibrating the cosmic distance ladder (using Cepheid variables, RR Lyrae stars)
• Determining absolute magnitudes of stars
• Studying stellar populations and galactic structure
• Measuring proper motions and space velocities

Galactic Structure and Dynamics

Kiloparsecs (kpc) describe structures within galaxies:

Milky Way structure:

• Galactic center (Sagittarius A*): 8.2 kpc from Sun
• Galactic disk radius: ~15 kpc
• Central bulge: ~1.5 kpc radius
• Spiral arms: trace patterns 10-15 kpc in radius
• Dark matter halo: extends to ~60 kpc

Rotation curves: Plot orbital velocity vs. distance (in kpc) from galactic center, revealing dark matter

Star formation regions: Giant molecular clouds span 10-100 pc

Globular clusters: Orbit 10-60 kpc from galactic center

Extragalactic Astronomy

Megaparsecs (Mpc) measure distances between galaxies:

Galaxy surveys: Map millions of galaxies to distances of 1,000+ Mpc, revealing large-scale structure (walls, filaments, voids)

Tully-Fisher relation: Links galaxy rotation speed to luminosity, enabling distance estimates in Mpc

Type Ia supernovae: Standard candles for measuring distances to 1,000+ Mpc

Galaxy clusters: Typical separation between major clusters ~10-50 Mpc

Superclusters: Structures spanning 100-200 Mpc (like Laniakea Supercluster containing Milky Way)

Cosmology and Universe Expansion

Megaparsecs and gigaparsecs describe cosmological distances:

Hubble constant (H₀): Measured in km/s per Mpc—describes universe expansion rate

• Current value: H₀ ≈ 67-73 (km/s)/Mpc (tension between measurement methods)
• Interpretation: Galaxy 1 Mpc away recedes at ~70 km/s; 100 Mpc away recedes at ~7,000 km/s

Hubble's Law: v = H₀ × d (where d is in Mpc, v is recession velocity)

Comoving distance: Cosmological distance accounting for universe expansion, measured in Mpc or Gpc

Redshift surveys: Map galaxy distribution to 1,000+ Mpc (z ~ 0.1-0.3 redshift)

Baryon acoustic oscillations: ~150 Mpc characteristic scale in galaxy distribution, used as "standard ruler"

Astrophysical Research Papers

Parsecs are the default distance unit in professional astronomy journals:

Observational papers: Report star/galaxy distances in pc, kpc, or Mpc

Theoretical models: Express scale lengths in parsecs (e.g., "disk scale length of 3 kpc")

Computer simulations: Use parsec-based units (or comoving kpc/Mpc for cosmological sims)

Standard convention: Professional astronomers think and calculate in parsecs, converting to light-years only for public communication

Conversion Guide

Converting Parsecs to Other Length Units

From parsecs (pc) to:

• Meters (m): pc × 3.0857 × 10¹⁶ = m
  • Example: 2 pc × 3.0857 × 10¹⁶ = 6.1714 × 10¹⁶ m
• Kilometers (km): pc × 3.0857 × 10¹³ = km
  • Example: 5 pc × 3.0857 × 10¹³ = 1.543 × 10¹⁴ km
• Miles (mi): pc × 1.9174 × 10¹³ = mi
  • Example: 3 pc × 1.9174 × 10¹³ = 5.752 × 10¹³ mi
• Light-years (ly): pc × 3.26156 = ly
  • Example: 10 pc × 3.26156 = 32.6156 ly
• Astronomical units (AU): pc × 206,265 = AU
  • Example: 0.5 pc × 206,265 = 103,132.5 AU

From kiloparsecs (kpc) to:

• Parsecs (pc): kpc × 1,000 = pc
• Light-years (ly): kpc × 3,261.56 = ly
  • Example: 8 kpc × 3,261.56 = 26,092 ly

From megaparsecs (Mpc) to:

• Parsecs (pc): Mpc × 1,000,000 = pc
• Light-years (ly): Mpc × 3,261,560 = ly
  • Example: 0.77 Mpc × 3,261,560 = 2,511,400 ly (Andromeda distance)
• Kilometers (km): Mpc × 3.0857 × 10¹⁹ = km

Converting to Parsecs

From light-years (ly) to parsecs:

• ly ÷ 3.26156 = pc
• Example: 100 ly ÷ 3.26156 = 30.66 pc

From astronomical units (AU) to parsecs:

• AU ÷ 206,265 = pc
• Example: 412,530 AU ÷ 206,265 = 2 pc

From kilometers (km) to parsecs:

• km ÷ 3.0857 × 10¹³ = pc
• Example: 9.27 × 10¹³ km ÷ 3.0857 × 10¹³ = 3 pc

From meters (m) to parsecs:

• m ÷ 3.0857 × 10¹⁶ = pc
• Example: 6.17 × 10¹⁶ m ÷ 3.0857 × 10¹⁶ = 2 pc

Parallax Angle to Distance

Most important conversion for astronomers:

Distance (parsecs) = 1 / parallax angle (arcseconds)

Examples:

• Parallax 0.5″ → 1/0.5 = 2 pc
• Parallax 0.1″ → 1/0.1 = 10 pc
• Parallax 0.01″ → 1/0.01 = 100 pc
• Parallax 0.001″ (1 milliarcsecond) → 1/0.001 = 1,000 pc = 1 kpc

For milliarcsecond (mas) measurements:

• Distance (pc) = 1,000 / parallax (mas)
• Example: 50 mas parallax → 1,000/50 = 20 pc

Common Conversion Mistakes

1. Confusing Parsecs with Light-Years

❌ Wrong: "Alpha Centauri is 1.3 light-years away" ✅ Right: "Alpha Centauri is 1.3 parsecs or 4.3 light-years away"

Why: Parsecs and light-years are similar but not equal: 1 pc ≈ 3.26 ly. Mixing them up introduces 3× error.

2. Incorrect Parallax-to-Distance Conversion

❌ Wrong: "A star with 0.5 arcsecond parallax is 0.5 parsecs away" ✅ Right: "A star with 0.5 arcsecond parallax is 1/0.5 = 2 parsecs away"

Why: Distance equals the reciprocal (1/p) of parallax angle, not the parallax itself.

3. Forgetting to Convert Milliarcseconds

❌ Wrong: "Gaia measured 50 milliarcsecond parallax, so d = 1/50 = 0.02 parsecs" ✅ Right: "50 milliarcseconds = 0.050 arcseconds, so d = 1/0.050 = 20 parsecs (or d = 1000/50 = 20 pc)"

Why: Must convert milliarcseconds (mas) to arcseconds first, or use the formula d(pc) = 1000/p(mas).

4. Misapplying Kiloparsec/Megaparsec Prefixes

❌ Wrong: "The Milky Way is 30 parsecs across" ✅ Right: "The Milky Way is 30 kiloparsecs (kpc) across, or 30,000 parsecs"

Why: Galaxy-scale distances require kiloparsecs; plain parsecs are far too small (off by 1,000×).

5. Mixing Up Parsec and Arcsecond

❌ Wrong: "Parsec is an angle measurement" ✅ Right: "Parsec is a distance measurement derived from parallax angle (arcsecond)"

Why: Arcsecond is an angle; parsec is a distance. The parsec equals the distance where 1 AU subtends 1 arcsecond.

6. Ignoring Light Travel Time vs. Distance

❌ Wrong: "Andromeda is 770,000 parsecs away, so we see it as it was 770,000 years ago" ✅ Right: "Andromeda is 770,000 parsecs (2.5 million light-years) away, so we see it as it was 2.5 million years ago"

Why: Light travel time corresponds to light-years, not parsecs. Must convert: 770,000 pc × 3.26 ly/pc = 2.5 million ly.